Takeda: I will ask Yuichi Moriguchi, who is carrying out investigations on radio-contamination caused by the accident, including the insoluble radioactive particles, how many of such insoluble radioactive particles exist and in what range of area?

Moriguchi: There are many different sizes of particles, but relatively large particles have been found only near the nuclear power plant. On the other hand, we know that the smaller particles were transported far by the wind and reached the Kanto region.

Kamakura: Please see here for the details.
Mr. Moriguchi and his colleagues have divided the insoluble radioactive particles into two major types. They are called type A and type B.
Those of type A are comparatively small with a size of 10 micrometers or less. A lot of them are spherical. What is called a cesium ball is of this type. Since they are small in size, these particles are likely to reach the lungs by breathing.
On the other hand, those of the type B are comparatively large, by more than several tens of micrometers, and most of them are of distorted shape. Because the particle is large, it is not possible to enter the lungs, but it may adhere to the skin and mucous membranes.

Kamakura: Please see here for the details.
Mr. Moriguchi and his colleagues have divided the insoluble radioactive particles into two major types. They are called type A and type B.
Those of type A are comparatively small with a size of 10 micrometers or less. A lot of them are spherical. What is called a cesium ball is of this type. Since they are small in size, these particles are likely to reach the lungs by breathing.
On the other hand, those of the type B are comparatively large, by more than several tens of micrometers, and most of them are of distorted shape. Because the particle is large, it is not possible to enter the lungs, but it may adhere to the skin and mucous membranes.

The areas where each type are scattered are gradually coming to be known.
A relatively large, heavy type B particle has been found within 20 kilometers of the Fukushima Daiichi nuclear power plant. On the other hand, small light type A particles are found in the Kanto region.

According to the simulation in the paper published by the meteorological laboratory, the type A particles were diffused like this by the wind on March 14-15 immediately after the accident.

Takeda: Smaller type A particles flew to the Kanto region immediately after the accident. Could you explain more?

Moriguchi: This is exactly what we are researching right now. The other day I presented a paper at an Academic society. We knew that radioactive materials had reached the Kanto area on March 15, but we found that there were insoluble radioactive particles among them. We are trying to clarify right now as to why they arrived there. We are coming to know gradually that the radioactive materials are likely to have been discharged at a certain time.

Takeda: Just for confirmation: these are the ones that flew in the period between March 14 and 15?

Moriguchi: Yes, that’s right.

Takeda: Do you have any estimation of the amount that has been transported in the wind?

Moriguchi: As a whole, I still don’t know how much has been scattered, but as for what flew to the Kanto area on March 15, we have the result of another research group, according to which 80% to 90% of the radioactive materials are composed of this insoluble type A particle. I think that it’s necessary to evaluate the influence carefully because it has reached a considerably large area from Fukushima Prefecture to the Kanto region.

Takeda: Mr. Kai, what is your opinion of the health effect of the A type?

Kai: In the case of radiation, there are external and internal radiation effects. According to the report of the United Nations Scientific Committee on the Effects of Atomic Radiation (UNSCEAR), the influence of external radiation is larger. Therefore, although it is necessary to review the effects of internal radiation due to the discovery of such insoluble particles, the over-all effects including external radiation do not change much, even when the effects of internal radiation have changed. However, the evaluation of internal radiation needs to be reviewed properly in any case.

Takeda: The UNSCEAR has evaluated that there is no health impact due to the amount of radiation in the metropolitan area. Is there a possibility that this evaluation is reversed?

Kai: In that sense, the influence of the internal radiation exposure will change, but I do not think that their evaluation will be revised, because it is assumed that the influence of external exposure is larger.

Kamakura: On the other hand, there are people who had been evacuated and recently returned to the vicinity of the nuclear power plant.
What are the reactions of the local governments about this insoluble radioactive particle?
For example, the environmental policy section of Okuma town says: “No special measures have been taken, but when people enter a difficult-to-return area, we tell them to wear a protective suit and a mask, and to be careful not to blow up the dust when cleaning the room.”
As you can see, all municipalities are basically dealing with the protective measures that have been carried out so far such as avoiding adhesion to the body and inhaling radioactive materials.

Takeda: Mr. Moriguchi, the evacuation orders have been lifted near the nuclear power plant, and some people have started to return. What are the points to be careful about?

Moriguchi: The decontamination work is done, and the evacuation orders are lifted because the radiation dose has dropped, but the fact is that the decontamination work was carried out only outdoors. Moreover, even in places where the radiation dose is comparatively low, there are areas where such radioactive particles entered residential rooms immediately after the accident. Therefore, I think it is necessary to take the radioprotection seriously.

Takeda: There is another problem that researchers are concerned about in the issue of insoluble radioactive particles. It is a problem called “re-scattering”, that is to say, particles are re-raised and scatted from the areas where decontamination has not been done, including the site of the nuclear power plant. In fact, a case of re-scattering was already observed in the past.
On August 19, 2013, at Fukushima Daiichi nuclear power plant, following the decommissioning plan, the debris removal work was on the way at the reactor #3. But… the radiation dose increased on the premises. The workers’ body pollution occurred.

At this time, Kyoto University’s research group observed an increase in atmospheric radioactive materials at a point about 26 kilometers away from the nuclear power plant. In addition, insoluble radioactive particles were collected at observation facilities between the nuclear power plant and the Kyoto University observation point.

The research group at Kyoto University simulated the scattering of radioactive particles based on the weather data of the day. As a result, it was learned that the particles that had been lifted in the debris removal work had scattered over a wide range and reached the observation point.

Takeda: What is your point of view about the health effect of this re-scattering?

Kai: I think that the dose is relatively small, but it is important to take the measurements properly and keep watching. I think that it is especially important to pay attention to measurement results of the round the clock dust monitors installed in the vicinity of the nuclear power plant.

Takeda: How about you, Mr. Moriguchi? What do you think of the measures to take against the problem of re-scattering?

Moriguchi: About the re-scattering, if a big problem happens, most probably it will be in connection with the decommission work. So this is the first thing to be careful about.

Takeda: Another thing: what are the effects of insoluble radioactive particles on the agricultural crops?

Moriguchi: They are actually monitored rigorously. The monitoring in the atmosphere is done as well as the rigorous control of farm products. I think that it is important to diffuse the information thoroughly.

Takeda: You mean that we can trust the products which are put in the market?

Moriguchi: I think that the monitoring is done well.

Takeda: Mr. Moriguchi and Mr. Kai are continuing the research to find out the range of the scattered particles, and also to evaluate the irradiation dose. They are hoping to have the results by the end of the fiscal year (the end of March).
Researchers are currently trying to clarify the risks of insoluble radioactive particles. And we are going to continue our investigations.
This may cause anguish to some people, but we think that it’s important to receive the information calmly for now.

We are presenting here a transcription of an NHK TV documentary (note1) on insoluble radioactive particles found in Fukushima and in the Tokyo metropolitan region. This is the 2nd part of the 3 parts.

Here is the first part.

Insoluble radioactive particles that do not dissolve in water.
This characteristic is supposed to make a big difference when considering health effects. In the past, radioactive cesium emitted in the nuclear accident was thought to be carried away adhering to water-soluble particles called aerosols in the atmosphere. When it touches the water the particle melts and the cesium diffuses and gets diluted. The same is true when it is inhaled in the lungs; the water-soluble cesium melts into the body fluid and spreads thinly throughout the body. Then it is supposed to be discharged gradually by the metabolic activity, and decreases by half from 80 to 100 days in the case of adults.

Insoluble radioactive particles, on the other hand, do not dissolve in body fluids. For example, if they adhere to the alveoli at the furthest areas of the lungs, it may take years to discharge. Even with the same amount of cesium, the dose of lung exposure is about 70 times higher than in the case of water-soluble cesium in the case of adults. As for the infants who are more radiosensitive, the dose of exposure is supposed to be approximately 180 times higher.

In fact, this insoluble radioactive particle has not been identified in past nuclear accidents. Why was it emitted in the accident of the Fukushima nuclear power plant?
Yukihiko Sato, who is doing research on this particle, is focusing on the insulation material that contains glass components. It is used in parts such as piping in the nuclear power plant.
A special electron microscope is used to analyze the proportion of elements contained in the radioactive particles and in the insulation material.
The top is radioactive particles, and the bottom is the insulation material. The proportion of elements, such as silicon and oxygen, which are the main components of glass, is well matched.

From this, Mr. Sato thought about the scenario where the radioactive particle formed as follows:
Radioactive cesium was emitted from the melted nuclear fuel in the event of the accident. It first filled the reactor. Then, it leaked into the reactor containment building.
Cesium was absorbed in the insulation material in the building.
After that, a nuclear reactor building blew up by hydrogen explosion. As the insulation material melts and becomes glass, cesium is taken in. And with the explosion, it became small particles as it dispersed in the blast.

The radioactive particles found by Sato are in diameter from 0.5 to 500 micrometer. Their shapes vary from a smooth round one to a rugged one.

Tatsuhiko Sato of the Japan Atomic Energy Agency. He simulated the health effects of insoluble radioactive particles using a program to calculate the behavior of each ray. For the simulation, he used a particle of the size which enters the lung, and which is actually found. He compared the simulation of the insoluble radioactive particles remaining to adhere to the same spot on the surface of the organ, and that of the same amount of radioactive material adhered uniformly on the surface.

In the case of uniform adhesion, even after 24 hours, blue and light blue areas are spread out indicating that the radiation dose is low.

On the other hand, in the case of the particle, the dose near the spot increases locally and orange and red areas are expanding.

Even with the same quantity of radioactive materials, the health effect may change.

In fact, there are data of people who may have inhaled insoluble radioactive particles. This is a survey of TEPCO employees who had a large amount of exposure during the nuclear accident.

The amount of the radioactive materials in the body is examined regularly, and the graph in red shows that the value of the vicinity of the chest is comparatively high. While the radioactive cesium that had spread throughout the body decreased over time, only around the chest the speed to decrease was slow. The inhaled insoluble radioactive particles are suspected to remain in the lungs.

However, researchers say that the amount is not significant enough to worry about the health effects, according to the International Commission on Radiological Protection.

Takeda: Mr. Michiaki Kai is a specialist in the radio-induced health damages and radioprotection.
If the insoluble radioactive particles stay in the body, the radiation dose may increase locally. And according to some experts, it is necessary to investigate the health effects. What is your opinion?

Kai: First of all, you know that the dose is a measure of health effects. However, when we compare the dose, you cannot compare the cases of smaller and larger exposures ranges. In general, the greater the exposures range, the greater the health impact is. In that sense, the larger the average dose of an organ or an entire system is, the greater the impact is. Therefore, it is important to evaluate the average organ dose even in the case of the insoluble particle. However, there is a possibility that the dose becomes high very locally, so it is important to evaluate it properly, since some people worry about it. This is why such an evaluation is carried out.

Takeda: The overall exposure more than local exposure is …

Kai: If it is the same dose, the impact on health is larger if the range of exposure is wider.

Takeda: You mean that the impact is larger, but it is also necessary to examine a local exposure.

A study published in March of 2016 found insoluble cesium lodged in the lungs of some of the highly exposed Fukushima disaster response workers. After doing additional scans they found most of that persistent cesium contamination resided in the workers lungs.

Direct measurements of seven highly exposed workers at the Tokyo Electric Power Company Fukushima Daiichi Nuclear Power Station accident have been performed continuously since June2011.

Caesium clearance in the monitored workers is in agreement with the biokinetic models proposed by the International Commission on Radiological Protection. After 500 days from the initial measurement, however, the caesium clearance slowed.

It was thought to be unlikely that additional Cs intake had occurred after the initial intake, as activity in foods was kept low. And, the contribution from the detector over the chest was enhanced with time. This indicates that insoluble Cs particles were inhaled and along metabolic rate showed.

The study concludes that insoluble cesium particles lodged in the workers lungs, preventing them from leaving the body through normal processes.

“The subjects seem to have inhaled insoluble caesium particles, even though it was only a small amount. At present, it is more plausible that the retention curves reported here were due to inhalation of a mixture of type F caesium (soluble particle) and type S caesium (insoluble particle).”

This finding is significant as it shows how the various kinds of insoluble radioactive cesium materials discovered after the initial disaster, both black substances and the glass spheres could contaminate the human body.